The present invention relates to video signal processing generally and, in particular, to the presentation of ordered dither images in an interlaced-field format.
Ordered dither, or more simply, "dither," is a video signal processing technique which provides bi-level display systems with gray-scale, or continuous-tone capability. At the heart of a bi-level display system is a display panel typically comprising a matrix of individual, closely spaced display cells each of which resides in one of two visual states. That is, each display cell is either completely energized (on) or completely de-energized (off). A plasma panel, for example, is one known type of bi-level display panel. Picture images and other graphic data are readily displayed on a bi-level display panel via selective energization of its cells.
Since the cells of a bi-level display panel are either completely on or completely off, the panel has no inherent capability for representing gray scale, i.e. varying intensities in reproduced images. Advantageously, however, the gray-scale information in an image can be approximated on a bi-level display panel by appropriate arrangement of on and off cells.
Dither processing is one known technique for determining which cells should be on and which off. The image to be reproduced is divided into a matrix of picture elements, each element corresponding to a respective cell of the display panel. A dither threshold value is assigned to each picture element from a predetermined "dither matrix." If the intensity of any given picture element is greater than the dither threshold value assigned thereto, the corresponding display cell is turned on. Otherwise it is maintained off.
Dithered image signals occupy substantially less bandwidth than other types of video signals such as standard television signals. The tradeoff is slightly reduced spatial resolution. However, for many video applications including face-to-face conversation, area surveillance, engineering and medical graphics exchange, cassette teaching systems and others, this tradeoff is most advantageous. For example, the narrow bandwidth occupied by a dither-processed image signal enables the use of regular telephone lines or other narrow-band media to transmit the image signal.
Although ordered dither was developed principally for use in bi-level display systems, it may be desirable to display dithered images on a conventional cathode-ray tube (CRT) display using the CRT in a bi-level mode. Such arrangements advantageously minimize both transmission bandwidth requirements and display hardware costs; at the present time. CRT displays are very much less expensive than plasma or other bi-level display panels.
A problem arises, however, because conventional CRT systems use an interlaced-field format. When a dithered image is displayed in this format, the two fields of each frame have unequal brightness because of the way in which the threshold values of a dither matrix are spatially distributed therein. The result is an annoying flicker in the displayed image. At least two techniques for minimizing this flicker are known. These techniques involve the modification or rearrangement of the dither threshold values within the dither matrix. One of the known techniques eliminates much but, disadvantageously, not all of the flicker. The other achieves better flicker reduction but, disadvantageously, only at a loss of substantial intensity resolution.